1. Field of the Invention
The present invention relates to a signal transmission device that is used to transmit a signal from a logic circuit through a transformer to another device.
2. Description of the Prior Art
Such a signal transmission device is used in a circuit that needs to meet strict requirements as to the overshoots and undershoots occurring in the circuits succeeding thereto, for example in a terminal adapter that is used to connect terminal equipment to an ISDN (integrated services digital network) line. As shown in FIG. 4, a conventional signal transmission device has, in its output stage, two pairs of serially connected P-channel and N-channel MOS transistors, QH1 and QL1 forming one pair and QH2 and QL2 forming the other pair, and an inverter circuit I for inverting the signal fed to the MOS transistors QH2 and QL2. The two ends of the primary coil L1 of a transformer are connected individually to the nodes K1 and K2 between the two transistors in those pairs. In a case where the primary coil L1 is driven so that a square wave will appear between the two ends of the secondary coil L2, the gate voltages A, B, C, and D of the individual transistors are varied as shown at (a) in FIG. 5; that is, all the transistors are turned on or off simultaneously.
In this conventional circuit configuration, however, since all the transistors are turned on or off simultaneously, large transient variations occur in the current flowing through the coil L1, and thus large back electromotive forces are induced by the inductance of the coil L1. As a result, as shown at (b) in FIG. 5, overshoots OS and undershoots US occur in the square wave E that appears between the two ends of the coil L2.
In particular, at the node (generally called the S/T point) between a terminal adapter, which is used to connect terminal equipment to an ISDN line, and a DSU (digital service unit), where an interface employing a coil is customarily used, the ISDN standards set a very strict requirements that overshoot and undershoot be within approximately 10 mV. This requirement, however, is very difficult to meet as long as the conventional circuit configuration is used.